This invention relates to power converters, and more particularly, to the kind known as series capacitor inverter-converters.
This type of converter utilizes internally series resonant circuits which link a source of D.C. energy with one or more loads simultaneously. These loads may require A.C. or D.C. power or a combination of both. The source energizes series resonant circuits in a succession of discrete time intervals. The oscillations of these series resonant circuits are limited to one half cycle duration since the circuits include controlled unidirectional electronic switches, such as thyristors. These devices are brought into a state of conduction following a trigger signal when the other conditions for forward current flow in this circuit exist. The thyristors block, however, current flow in the reverse direction; the resonant circuit comes, therefore, to rest after one half cycle of its natural period of oscillation. Triggering of a second appropriately placed thyristor after elapse of some time interval causes the occurrance of another half cycle of resonant oscillation. After some time the first thyristor is triggered, and the process goes on in the described manner.
Energy is extracted from the resonant circuit during each of the described cycles via a transformer. An output filter smoothes the succession of pulses to accomodate the load requirements. The described extraction of energy has a damping effect on the, otherwise, almost undamped oscillations in the series resonant circuits. The current amplitude in these circuits depends on the (1) D.C. voltage of the source of energy, (2) the damping caused by the load and (3) the quality Q of the entire resonant circuit. If any of these three aspects of the oscillating system departs from an intended and workable state, then the circuit will either (a) cease to oscillate for want of sufficient acceptance of energy, or (b) its current amplitude may increase to levels where the ohmic losses in the circuit could become prohibitive and, possibly, damaging because of the ensuing heating effects, or the circuit elements could be destroyed by excessive voltage stresses.
The above described conditions are prevalent in the discussed kind of power converters, especially if the converter is required to supply constant power to a load during appreciable changes of the D.C. source voltage. An analogous condition develops when the load voltage is appreciably varied at a time when the D.C. supply voltage is kept constant.